1. Field of the Invention
This invention relates to an apparatus for remotely applying radiant heat across a curved section of a metallic tube from the interior of the tube. It is particularly useful in heat treating the U-bend sections of heat exchange tubes in order to stress-relieve them.
2. Description of the Prior Art
Devices and processes for applying heat to the inside surfaces of tubes are known in the prior art. Such devices are frequently used to braze reinforcing sleeves within the U-shaped heat exchange tubes of nuclear steam generators. One such device generally comprises a mandrel integrally formed from a rigid, heat-resistant ceramic material, such as boron nitride. The central portion of the mandrel includes a set of spiral grooves for receiving and spacing apart an electrical resistance heating element, which may be a platinum-rhodium wire. The ends of such mandrels each include enlarged shoulders for concentrically spacing the coiled heating element from the inner walls of the tube. An example of such a radiant heater is described and claimed in U.S. patent application Ser. No. 634,336, filed July 13, 1984 by John M. Driggers and assigned to the Westinghouse Electric Corporation, the entire specification of which is hereby expressly incorporated herein.
In operation, the elongated cylindrical body of the heater is inserted into the open end of a tube, and slidably positioned adjacent to a region of the tube where a brazing heat needs to be applied. Typically, such heaters are used to braze in place a cylindrical reinforcing sleeve which has been inserted into a heat exchange tube across a section of the tube where the outer walls have been attacked by corrosion. When the proximal and distal ends of such sleeves are brazed to the inner walls of the tube, a fluid bridge is formed across the corroded section of the tube. In order to position the heater to a desired location within the tube or sleeve, a nylon push rod attached to the proximal end of the mandrel is used. A specific process for positioning such a heater to effect such braze joints is described and claimed in the aforementioned patent application Ser. No. 634,336.
Unfortunately, such prior art tube heaters are not well adapted for use along the more tightly curved U-bend sections of the heat exchange tubes in nuclear steam generators. While it may be possible to slidably position such heaters along the U-bend sections of such tubes having a large radius of curvature, the rigid, elongated mandrel simply cannot negotiate tubes having small-radiused U-bend sections. Such small-radiused U-bend sections are present in the central portion of the tube plate of nuclear steam generators. Here, the U-bend radii may be as short as two inches. The inability to negotiate such U-bends is a significant limitation, since it has been recently discovered that such small-radiused U-bend sections frequently contain a sustantial amount of residual tensile stress which can cause these sections of the tubes to degrade and crack as a result of the well-known phenomenon of stress-corrosion cracking. Such cracking can, of course, cause the nonradioactive water in the secondary system of the steam generator to become radioactively contaminated by the hot, radioactive water which flows through the primary system of the steam generator. This, in turn, will cause the steam which drives the electric turbines to become radioactively contaminated. While such residual tensile stresses may be relieved by annealing these tightly curved sections of the centrally disposed heat exchange tubes, prior art tube heaters having rigid mandrel structures offer no convenient and practical way to apply annealing heats to these sections.
Tube heaters having some degree of flexibility are also known in the prior art. However, these heaters are likewise ill-adapted to heat treat such U-bends. Such a tube heater is exemplified by U.S. Pat. No. 4,532,396 by Robert D. Burack et al. and assigned to the Westinghouse Electric Corporation. While this particular tube heater is capable of producing high quality braze joints in the sections of the heat exchange tubes close to the tubehseet, power transmission problems would arise if one attempted to use this induction-type heater twenty to thirty feet above the tubesheet where the U-bend sections of the tubes are located. Specifically, because only about one-twentieth of the power applied to such a heating coil would ultimately be transferred to the U-bends, a very large and expensive power source would be necessary for such a heater to heat treat the U-bend of a tube in a short time. Since generator downtime is very expensive, any annealing device which cannot heat treat these U-bends within a few minutes time would simply be impractical. Finally, such tube heaters are simply not sufficiently flexible for use within the smaller U-bend radii.
Clearly, there is a need for a tube heater capable of quickly, conveniently and inexpensively applying heat across the U-bend sections of the heat exchange tubes of nuclear steam generators in order to relieve the residual stresses therein. Ideally, such a heater should be quickly and easily slidable up the straight leg portion of the tubes (which may be 30 feet long) and into the U-bend portion thereof and should be operable through a small and inexpensive power source. Such a heater should also have the ability to quickly expose a large portion of a small-radiused U-bend section to an annealing heat in order to minimize the time necessary to complete the annealing operation. Finally, such a tube heater should be extremely reliable in operation and durable in construction so that it can quickly anneal a large number of small-radiused U-bend sections before it needs to be replaced.